Sunday, 26 May 2013

Keeping the heat on, or powering Europe!

 Moving on to Toronto, Florence, Vladivostok or Seattle?

Toronto, Florence, Vladivostok and Seattle are particular cities with particular surroundings. All four cities are situated about half way between the Equator and the North-pole, but what for a different climate they offer! Definitely, it's not the same choice where to live. 

Toronto Skyline
This four cities are situated at about the same northern latitude. Toronto in eastern Canada is facing the cold western North Atlantic Ocean.  
Florence in Europe is facing the warm eastern North Atlantic Ocean. Vladivostok in the far east of Russia is situated at the cold western shores of the Pacific Ocean, and Seattle at its  warm eastern shores.

Florence and Seattle have warm summers and temperate winters. Toronto and Vladivostok have temperate summers and really cold winters. Florence has the most clement climate, followed by Seattle, Vladivostok coming well last. 

Florence annual mean day-time temperature is 20°C, and its annual mean night-time temperature is 9°C. For Seattle the annual mean temperatures are 15°C and 7°C, respectively.  Toronto has an annual mean day-time temperature of 13°C, and its annual mean night-time temperature is 5°C. In Vladivostok the annual mean day-time and night-time temperatures are 9°C and 2°C, respectively.  

The average day in Vladivostok is as warm as the average night in Florence!

Watch your neighbouring ocean

These differences of the local climate of  Toronto, Florence, Vladivostok or  Seattle are strongly determined by the temperature of the surface waters of their neighbouring ocean. The  surface waters at northern east-coast of the North-Atlantic Ocean are much warmer than surface waters as its northern west-coast. The same pattern is found in the North-Pacific; the northern east-coast of  North Pacific is much warmer than its northern west-coast. The cross-ocean temperature difference is more pronounced in the North Atlantic Ocean than in the North Pacific Ocean. 

Florence -  Arno River
Lucky Europe, is heated by the currents of the North Atlantic Ocean!  Not so lucky Japan, its northern islands are cooled by the Pacific Ocean but happily Japan's southern islands are swept by the warm Kurishio. The currents of the Kurishio does for the North Pacific Ocean and North America what the Gulf Stream does for the North Atlantic and Europe; warm surface waters are pushed north-eastward across the ocean. 

Kurishio and Gulf Stream are part of a much wider pattern of  variable global currents and related fluctuating transport of heat, salt [1] and other substances.

A kind of global 'conveyor belt'  links the oceans at the top and at the bottom, with surface currents transporting warm water northward to the Arctic while cold water in the depths flows back to the tropics and around the world. But that belt operates with "stop and go", with the strength of currents varying widely from year to year, decade to decade.  

This circulation, in the North Atlantic called the 'Atlantic Meridional Overturning Circulation' ferries vast amounts of heat from the tropics to northern latitudes [2]. One of its main components is the Gulf Stream. But far more is happening below the surface in the depth of the ocean that determines features and variable strength of  currents and related heat transport [3]. The 'Atlantic Meridional Overturning Circulation' forms the part of the global  'conveyor belt' that operates in the North Atlantic Ocean.

Warm conveyor belt waters from Cape Hatteras to Murmansk

Global Conveyor Belt
The Gulf Stream transports heat along the eastern shores of North America up to Cape Hatteras and than into the wider North Atlantic Ocean setting of as the North Atlantic Current. The warm surface water moves across the North Atlantic Ocean to Europe and up into the Greenland Sea between Norway and Greenland. There part of the warm water is cooled and sinks into the depth, and part moves further north into the Arctic Ocean keeping the very northern European ports (at 69° N) ice-free. So regular cargo-ships can sail to Kirkenes and Murmansk at latitudes where elsewhere at similar latitude only icebreakers may navigate the ocean. The warm sea heats the western winds that keep Europe's climate mild. Nowhere else in the northern hemisphere is the climate so clement at the same northern latitudes.

The North Atlantic Ocean is the powerhouse heating Europe.

Conveyor Belt in the North Atlantic Ocean
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Year-to-year and longer term changes in the strength of the  'Atlantic Meridional Overturning Circulation' happen and likely are affecting seasonal wetter conditions across Europe, Africa and the Americas.

For example, observations from 2009 indicate that strength of the overturning circulation dropped by 30% for a year. This reduced the amount of heat transported to the North Atlantic by almost 200 trillion watts. That drop of heat transport into the North Atlantic ocean has been linked to the harsh winters in Europe 2009-10 [4]. 

The estimate drop of heat transport is only a minor part of the total heat transport around the globe by ocean currents, nevertheless  200 trillion watts  is a tremendous amount of heat. This amount of heat corresponds to about half of  the additional amount of heat that currently  is captured by the atmosphere because of the increased carbon dioxide concentration of the atmosphere;  increase beyond the pre-industrial values of 290 ppm currently we hit the 400 ppm [ppm = parts per million]. A fluctuating heat tarnsport of that size is important. Therefore detailed and lasting observation along transects of the entire Atlantic Ocean are planned for the next years [4].

Between Greenland and Caribbean Seas

Cape Hatteras 
A key element of the conveyor belt is found in the the Greenland Sea and the Labrador Sea. When reaching this region in the north-eastern North Atlantic Ocean the warm surface waters cool and sink; sink really deep down to the bottom of the sea [5]. In that cooling process heat is passed on to the atmosphere and carbon dioxide is carried to the depths, sequestering it from the atmosphere.  Winds move warmed air eastward into Europe.

Water in the depth is moving back towards the equator along the continental slopes of Greenland and North America. Thus a cold southward current runs in the depth of the ocean along the North American east cost. The deep current is  accompanied by cold surface waters, the Labrador current, that sweeps the shores of Canada and north-eastern states of the USA before - south of Cape Hatteras - the warm northward flowing Gulf Stream dominates the surface currents. Thus, the east-coast of North America is cooled in the north and heated in the south. However, in the depth all along  the continental slope a mighty vein of cold water runs southward, the back-loop of the 'conveyor belt' in the North Atlantic.

The cold water that was formed in European sup-polar seas moves south into the South Atlantic Ocean where waters from different sources meet, including water from the Indian Ocean and the Pacific Ocean. The South Atlantic Ocean has its own less vigorous overturning circulation. It is moving heat along the shores of South America from the tropics poleward linking into the mighty circumpolar current sweeping around the Antarctic continent. Likewise - as a key part of the global 'conveyor belt' -  heat is swept  by surface currents northward out of the tropical South Atlantic Ocean. Warm water flows into the western tropical North Atlantic Ocean and the Caribbean Sea from where after further heating the even warmer water is shifted forward to Europe.

"What would happen if the North Atlantic Current
should stop or change direction?"... climate in Labrador and Ireland. 
The net global oceanic heat transport is into the North Atlantic where the heat is released from the ocean into the atmosphere. This heat then is swept by the winds into Europe, Northern Africa and Asia.

The relative warm surface waters of the North Atlantic Ocean, which are already enriched in salt content for example by the outflow of salty water from the Mediterranean, pass humidity to the warm atmosphere. The surface waters get saltier than surface waters of any other ocean. This high salt content is a favourable precondition for the deep-water formation in  Greenland Sea and Labrador Sea, what in turn is engine moving around the 'conveyor belt'. That engine drives the North Atlantic powerhouse for keeping Europe's climate warm and clement.

[1] See related posts: 
[2] The similar feature does not occur in the Pacific Ocean with the same strength because the Bering Sea linking it with the Arctic Ocean is shallow and the salt content of the surface waters of the Pacific Ocean is lower. However the winter freezing of Okhotsk Sea causes formation of a modest volume of deep water.

[3] McCarthy, G. et al. (2012), Observed interannual variability of the Atlantic meridional overturning circulation at 26.5°N, Geophysical Research Letters: "The Atlantic meridional overturning circulation (MOC) plays a critical role in the climate system and is responsible for much of the heat transported by the ocean. A mooring array, ...  provides continuous measurements of the strength and variability of this circulation. With seven full years of measurements, we now examine the interannual variability of the MOC. While earlier results highlighted substantial seasonal and shorter timescale variability,... From 1 April 2009 to 31 March 2010, the annually averaged MOC strength was just 12.8 Sv[erdrup = 1.000.000 m³ / second], representing a 30% decline. This downturn persisted from early 2009 to mid-2010....  This rebalancing of the transport from the deep overturning to the upper gyre has implications for the heat transported by the Atlantic."

[4] Q. Schiermeier, Ocean under surveillance, Nature, Vol. 497 p.167-168. The article by Q. Schiermeier has motivated me to prepare this contribution for my blog "Mundus Maris... first news". 

[5] Rudels, B. Quadfasel D. (1991), Convection and deep water formation in the Arctic Ocean-Greenland Sea System, Journal of Marine Systems: "The processes of convection and deep water formation in the Nordic Seas.... The dense shelf waters sink on the continental slopes into the deep basins entraining ambient waters from the strongly stratified Arctic Ocean proper. In the European Polar Seas—the Nordic Seas—deep water is only formed in the Greenland Sea through haline convection, ...resulting in a weakly stratified water column...".

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